The field of wireless communications is expanding at a phenomenal rate, as more radio spectrum becomes available for commercial use and as cellular phones become commonplace. In addition, there is currently an evolution from analog communications to digital communications. In digital communications, speech is represented by a series of bits which are modulated and transmitted from transmitter (e.g.,a base station) to a receiver (e.g.,a mobile phone). The receiver demodulates the received waveform to recover the bits, which are then converted back into speech. There is also a growing demand for data services, such as e-mail and Internet access, which require digital communications.
There are many types of digital communications systems. Traditionally, frequency division-multiple-access (FDMA) is used to divide the spectrum up into a plurality of radio channels corresponding to different carrier frequencies. These carriers may be further divided into time slots, a technique referred to as time-division-multiple-access (TDMA), as is done in the D-AMPS, PDC, and GSM digital cellular systems.
If the radio channel is wide enough, multiple users can use the same channel using spread spectrum (SS) techniques and code-division-multiple-access (CDMA). IS-95 and JSTD-008 are examples of CDMA standards. With direct sequence spread spectrum (DS-SS), information symbols are represented by sequences of symbols referred to as chips. This spreads the information symbols in the frequency band. At the receiver, correlations to the chip sequences are used to recover the information symbols. Spreading allows the system to operate at a low chip signal-to-noise ratio (SNR). By choosing spreading codes with good auto-and cross-correlation properties cross talk between different users can be kept at a low level allowing multiple user signals to occupy the same bandwidth at the same time.
The radio signal is reflected and scattered off of various objects, giving rise to multi-path propagation. As a result, multiple images of the signal arrive at the receive antenna. When these images have roughly the same delay, relative to the chip period, they give rise to fading. Fading occurs because the images add sometimes constructively, and sometimes destructively. When these images arrive with different delays relative to the chip period, they can be viewed as echoes of the signal and are often referred to as "resolvable multi-paths," "rays," or simply "multi-paths."
To communicate efficiently and reliably, the receiver should exploit the multi-path fading channel by collecting signal energy from the different multi-paths. This is achieved by employing a RAKE receiver, which individually detects each echo signal using a correlation method, corrects for different time delays, and combines the detected echo signals coherently. The RAKE receiver includes a number of processing RAKE branches or "fingers." Using a delay searcher, the receiver searches for delays of the multi-paths and assigns an estimated delay to each one of the RAKE branches. Each RAKE branch then despreads the signal received over a path with a corresponding delay. The RAKE branch outputs are RAKE combined by weighting them and adding them together.
For mobile communications, the movement of mobile stations changes multi-path delays over time. To maintain performance, the delay estimation procedure must be able to track the multi-path delays. In conventional tracking RAKE receivers, RAKE branches are equipped with corresponding tracking devices, which employs delay tracking techniques, such as the early/late gate (ELG) and tau-dither techniques. With these delay tracking techniques, the signal energy is measured slightly before and slightly after the estimated delay. When the estimated delay is correct, then the early and late measurements should be approximately equal, as the chip pulse waveform falls off symmetrically about its peak. When an imbalance is detected, the delay estimate is adjusted to restore balance.
In a DS-CDMA based system, the ELG technique is implemented using two independent correlation receivers, an early correlation receiver and a late correlation receiver. Each correlation receiver works with a spreading code, also known as pseudo-noise (PN) code, that is shifted plus and minus a fraction k of the chip period T.sub.C relative to the estimated delay used by the RAKE branch. In order to adjust for path delay changes, the estimated received power from the early and late correlation receivers are compared, usually low-pass filtered, and used to control the phase of a local PN code generator.
As explained above, in conventional CDMA receivers, each RAKE branch has a dedicated tracking device. Because of the signal processing requirement, the implementation of a dedicated tracking device for each RAKE branch significantly complicates the hardware design of the CDMA receiver. For example, implementation of dedicated ELGs, each having two correlation receivers, for each one of the RAKE branches, requires twice as many ELG correlation receivers as data demodulating correlator receivers. In addition to complex hardware requirement, the implementation of dedicated tracking devices also increases the CDMA receiver's power consumption. Therefore, there exists a need to reduce hardware complexity and power consumption of the CDMA receivers that utilize tracking RAKE receivers.